Methods and materials for identifying novel pesticide agents

ABSTRACT

Materials and methods for identifying novel pesticide agents are disclosed herein. Specifically exemplified is a full length aminopeptidase N isolated from  Manduca sexta , insect cells expressing APN, and methods of screening pesticide agents using the same. Also disclosed are methods of identifying novel APN inhibitors.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 09/657,931, filed Sep. 7, 2000, now U.S. Pat. No. 6,586,197 which claims the benefit of U.S. Provisional Application Ser. No. 60/153,116, filed Sep. 7, 1999.

This invention was made in part with government support under Grant No. NIH AI29092 awarded by the National Instituted of Health and under Grant Nos. USDA/NRI 95-37302-1803 and 95-37302-4548 awarded by the U.S. Department of Agriculture/National Research Initiative. The government may have certain rights in this invention.

BACKGROUND OF THE INVENTION

Aminopeptidase N (APN) is an exopeptidase that hydrolyses neutral amino acids from the amino (N)-termini of different proteins. In different cell types, APN is expressed as a soluble cytoplasmic enzyme and a membrane-bound ectoenzyme. This enzyme is found on the surface of diverse cell types including lung, kidney, intestine and brain cells of many animals (Kenny et al., 1987). The ectoenzyme form is attached to epithelial cells of intestinal brush borders and respiratory tracts of vertebrates by a hydrophobic N-terminal stalk (Kenny et al., 1987 and Takasaki et al., 1991). In insects, however, ectoenzyme attachment is via a glycosyl-phosphatidylinositol (GPI) anchor (Tomita et al., 1994; Garczynski and Adang, 1995; Luo et al., 1996a; Luo et al., 1996b; Luo et al., 1997a; and Luo et al., 1997b). GPI-anchored proteins are relatively mobile on the membrane surface and can be clustered in microdomains with other proteins and specific lipids. The base of the GPI-anchor interacts with the intracellular environment and has been implicated in physiological functions, intracellular sorting and transmembrane signaling (McConville and Ferguson, 1993).

In intestinal epithelial cells, APN is important for the final hydrolysis step of ingested proteins. APN also has several important physiological roles in other tissues. For example, APN is implicated in tumor cell invasion and inhibition of aminopeptidase activity can suppress tumor cell spread (Fujii et al., 1995). In brain cells, APN serves a role in the breakdown and inactivation of peptide neurotransmitters (Kenny et al., 1987). In bovine renal brush border membrane vesicles (BBMV), partially purified APN was found to be associated with a Na⁺-dependent amino acid co-transporter (Plakidou-dymock et al., 1993).

APN molecules function as adventitious receptors for viruses. Human, feline, canine, and porcine coronaviruses utilize APN as their cellular receptors (Delmas et al., 1992; Yeager et al., 1992; and Tresnan et al., 1996). Cells refractory to coronaviruses from a particular animal species can be made susceptible by expression of an APN cDNA from that species (Benbacer et al., 1997). Human APN was shown to mediate human cytomegalovirus infection by increasing virus binding (McLaughlin and Aderem, 1995). Human, porcine and feline APNs have been cloned and expressed in different cell lines (Delmas et al., 1992; Yeager et al., 1992; Kolb et al., 1996; and Tresnan et al., 1996). Each of these vertebrate APNs were expressed on the cell surface as the N-terminal stalked form and bound a coronavirus.

Isoforms of APN located in the epithelial cells of insect midguts bind specifically to Bacillus thuringiensis Cry1 δ-endotoxins. Toxin-binding APNs are reported for several lepidopteran species (see, e.g., Knight et al., 1994; Sangadala et al, 1994; Gill et al., 1995; Valaitis et al., 1995; Luo et al., 1996; and Yaoi et al., 1997). For example, Cry1Aa, Cry1Ab and Cry1Ac, but not Cry1C or Cry1E toxins bind to a purified 115 kDa APN from Manduca sexta (Masson et al., 1995). Also partially purified preparations of APN catalyze toxin-induced pore formation in membrane vesicles (Sangadala et al., 1994) and planar lipid bilayers (Schwartz et al., 1997).

Several APN isoforms have been purified and cloned from different insect species (see, e.g., Knight et al., 1995; Gill et al., 1995; Valaitis et al., 1995; Luo et al., 1996; Yaoi et al., 1997; Denolf et al., 1997; and Hua et al., 1998). However, there has been limited success in expressing insect APN cDNA in insect cells. The only example to date involved the expression of Plutella xylostella 105 kDa APN in Sf9 cells using a baculovirus vector (Denolf et al., 1997). While the transformed cells of this study produced APN localized to the cell membrane, the APN was unable to bind to B. thuringiensis Cry1A toxins. Further, Denolf et al. were unsuccessful in expressing two 120 kDa APNs from Manduca sexta using the same vector.

The complete structural and functional characterization of insect APN will require the successful expression of insect APN in insect cells. Successful expression of insect APN in insect cells as described in Luo et al. (1999) would also facilitate study of APN-toxin interactions, as well as provide a screening system for obtaining novel pesticide agents.

BRIEF SUMMARY OF THE INVENTION

The subject invention pertains to cells expressing a polynucleotide encoding an Aminopeptide N(APN), and methods of using the same to identify pesticide agents. One aspect of the invention pertains to an isolated polynucleotide which encodes a full length APN from Manduca sexta (M. sexta) (SEQ ID NO: 1). Another aspect pertains to a fragment of said full length polynucleotide which is sufficient to encode a functional protein.

In another aspect, the subject invention pertains to a cell or cells transfected with a polynucleotide encoding an APN or fragments thereof, such that a functional polynucleotide is expressed. Preferably, the polynucleotide is expressed forming a protein which is localized at the cell membrane of said cell or cells. A further aspect pertains to descendent generations of said cells which express APN that is localized on the cell membrane.

In a further aspect, the subject invention is directed to a method of identifying pesticide agents comprising obtaining a cell or cells transfected with a polynucleotide encoding an APN or fragments thereof, such that said polynucleotide is expressed to produce a protein localized at the cell membrane of said cell or cells, and screening one or more pesticide agents for their ability to produce an observable effect on said cell or cells.

In yet another aspect, the subject invention is drawn to novel pesticide agents obtained according to the subject methods.

In a still further aspect, the subject invention is drawn to an expression vector comprising a polynucleotide encoding APN or a functional fragment thereof.

An alternative aspect of the subject invention pertains to a method of identifying novel aminopeptidase inhibitors comprising obtaining cells having APN localized on the cell membranes thereof; and screening one or more compounds of interest for their ability to inhibit aminopeptidase activity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents construction of the expression vector pHSP120.

FIGS. 2A–2E represent the amino acid/polynucleotide sequence of the APN1a from M. sexta (SEQ ID NO: 1 and SEQ ID NO: 2, respectively).

FIGS. 3A–3C represent a comparison of the cloned Manduca Sexta 120 kDa APN1a (M. Sexta 1a)(SEQ ID NO:1) with M. sexta 120 kDa APN2 (M. sexta 2)[Denolf et al., 1997](SEQ ID NO: 9), Plutella xylostella APN (P. xylostella)[Denolf et al., 1997](SEQ ID NO: 10), M Sexta 120 kDa APN1 (M. sexta 1)[Knight et al. 1995](SEQ ID NO: 11), Bombyx mori APN (B. mori)[Hua et al., 1998](SEQ ID NO:12), and Heliothis virescens APN (H. virescens)[Gill et al. 1995](SEQ ID NO: 13).

FIG. 4 represents immunoblot analysis of M. sexta 120 kDa APN expressed in Sf21 cells and 115 kDa APN purified from M. sexta midguts.

FIG. 5 represents autoradiograph of in vitro transcription and translation using various plasmid DNAs.

FIGS. 6A–6B represent optimization of expression of M. sexta 120 kDa APN in Sf21 cells.

FIGS. 7A–7B represent immunoblot analysis of M. sexta 120 kDa APN using anti-CRD antiserum.

FIGS. 8A–8D represent immunofluorescence localization of M. sexta 120 kDa APN1a in Sf21 cells by confocal microscopy.

FIG. 9 shows APN activity of Sf21 cells transfected with pHSP120 (columns A and B) or pHSP-HR5 (columns C and D).

FIGS. 10A–10D represent immunofluorescence analyses of the binding of Cry1Ac toxin and anti-APN antibody to Sf21 cell surface by confocal microscopy.

FIGS. 11A–11D represent immunofluorescence analyses of the binding of Cry1Ba toxin (55 kDa form) and anti-APN antibody to Sf21 cell surface by confocal microscopy.

FIGS. 12A–12D represent immunofluorescence analyses of the binding of Cry1Ba toxin (65 kDa form) and anti-APN antibody to Sf21 cell surface by confocal microscopy.

FIG. 13 represents immunoblot analysis of Cry1Ac, Cry1Ba and Cry3 toxin-affinity column purified M. sexta 120 kDa APN from Sf21 cells and from M. sexta midguts.

FIG. 14 represents cytotoxicity of Cry1Ba (55 kDa form) to Sf21 cells transfected with pHSP120 (open circle) and pHSP-HR5 (closed circle).

BRIEF DESCRIPTION OF THE SEQUENCES

SEQ ID NO: 1) is the amino acid sequence for the APN1a from M. sexta.

SEQ ID NO: 2) is the nucleotide sequence for the APN1a from M. sexta.

SEQ ID NO: 3) is the nucleotide sequence for PCR primer MS1.

SEQ ID NO: 4) is the nucleotide sequence for PCR primer MS1R.

SEQ ID NO: 5) is the nucleotide sequence for PCR primer MS5R.

SEQ ID NO: 6) is the nucleotide sequence for PCR primer MS4.

SEQ ID NO: 7) is the nucleotide sequence for vector primer T3.

SEQ ID NO: 8) is the nucleotide sequence for vector primer T7.

SEQ ID NO: 9) is the amino acid sequence for the M. sexta 120 kDa APN2 (Denolf et al. 1997).

SEQ ID NO: 10) is the amino acid sequence for the P. xylostella APN (Denolf et al. 1997).

SEQ ID NO: 11) is the amino acid sequence for the M. sexta 120 kDa APN1 (Knight et al. 1995).

SEQ ID NO: 12) is the amino acid sequence for the B. mori APN (Hua et al. 1998).

SEQ ID NO: 13) is the amino acid sequence for the H. virescens APN (Gill et al. 1995).

SEQ ID NO: 14) is the nucleotide sequence for the PCR primer 5′-pAHR5.

SEQ ID NO: 15) is the nucleotide sequence for the PCR primer 3′-pAHR5.

DETAILED DISCLOSURE OF THE INVENTION

As noted above, the subject invention relates to a cell or cells transfected with a polynucleotide encoding an APN protein, or fragment thereof, and methods using the subject cells for identifying novel pesticide agents. The subject invention provides, for the first time, insect cells that express a B.t. toxin binding aminopeptidase localized at their cell membrane. Further, the subject invention is the first demonstration of cultured cells which express a protein capable of binding to a toxin. Further, binding to the toxin is capable of producing an observable effect on such cells, including effecting the death of such cells.

In one embodiment, the subject invention is drawn to a polynucleotide that encodes an APN from M. sexta. In a preferred embodiment, the polynucleotide of the subject invention comprises a nucleotide sequence as shown in FIGS. 2A–2E (SEQ ID NO: 2).

In another embodiment, the subject invention is drawn to a cell or cells transfected with a polynucleotide molecule that comprises a nucleotide sequence encoding an APN protein or fragment thereof, wherein said APN protein or fragment thereof is localized at the cell membrane of said cell or cells. Further, the APN protein or fragment thereof is preferably, but not necessarily, anchored to said cell membrane by at least one glycosyl-phosphatidyl inositol anchor. In a preferred embodiment, said APN protein or fragment thereof which is localized at the cell membrane is capable of binding a toxin. In a more preferred embodiment, said APN protein or fragment thereof mediates an observable toxicity to said cell or cells, including death upon contacting a toxin.

The term “transfection” as used herein means an introduction of a foreign DNA or RNA into a cell by mechanical inoculation, electroporation, agroinfection, particle bombardment, microinjection, or by other known methods.

The term “transformation” as used herein means a stable incorporation of a foreign DNA or RNA into the cell which results in a permanent, heritable alteration in the cell. Accordingly, the skilled artisan would understand that transfection of a cell may result in the transformation of that cell.

As described in the background of the invention, many B.t. toxins have been isolated and sequenced. Polynucleotides encoding any known B.t. toxins or those yet to be discovered and active fragments thereof (see, for example, U.S. Pat. No. 5,710,020) can be used in accord with the teachings herein. These include, but are not limited to, polynucleotides encoding Cry1Aa, Cry1Ab, Cry1Ac, Cry1B, Cry1C, Cry1E, and Cry3A. See Crickmore et al. (1998) for a description of other Bt toxins.

As used hereinafter, “APN” includes full-length APN and fragments of APN operable for the uses disclosed herein.

In order to provide an understanding of a number of terms used in the specification and claims herein, the following definitions are provided.

An isolated nucleic acid or polynucleotide is a nucleic acid the structure of which is not identical to that of any naturally occurring nucleic acid or to that of any fragment of a naturally occurring genomic nucleic acid spanning more than three separate genes. The term therefore covers, for example, (a) a DNA which has the sequence of part of a naturally occurring genomic DNA molecule but is not flanked by both of the coding or noncoding sequences that flank that part of the molecule in the genome of the organism in which it naturally occurs; (b) a nucleic acid incorporated into a vector or into the genomic DNA of a prokaryote or eukaryote in a manner such that the resulting molecule is not identical to any naturally occurring vector or genomic DNA; (c) a separate molecule such as a cDNA, a genomic fragment, a fragment produced by polymerase chain reaction (PCR), or a restriction fragment; and (d) a recombinant nucleotide sequence that is part of a hybrid gene, i.e., a gene encoding a fusion protein. Specifically excluded from this definition are nucleic acids present in mixtures of (i) DNA molecules, and (ii) cell clones, e.g., as these occur in a DNA library such as a cDNA or genomic DNA library.

A nucleotide sequence is operably linked when it is placed into a functional relationship with another nucleotide sequence. For instance, a promoter is operably linked to a coding sequence if the promoter effects its transcription or expression. Generally, operably linked means that the sequences being linked are contiguous and, where necessary to join two protein coding regions, contiguous and in reading frame. However, it is well known that certain genetic elements, such as enhancers, may be operably linked even at a distance, i.e., even if not contiguous.

Polynucleotide probes include an isolated polynucleotide attached to a label or reporter molecule and may be used to identify and isolate other APN-encoding polynucleotides. Probes comprising synthetic oligonucleotides or other polynucleotides may be derived from naturally occurring or recombinant single or double stranded nucleic acids or be chemically synthesized. Polynucleotide probes may be labeled by any of the methods known in the art, e.g., random hexamer labeling, nick translation, or the Klenow fill-in reaction.

The polynucleotides may also be produced by chemical synthesis, e.g., by the phosphoramidite method described by Beaucage and Caruthers (1981) Tetra. Letts., 22:1859–1862 or the triester method according to Matteuci et al. 91981) J. Am. Chem. Soc., 103:3185, and may be performed on commercial automated oligonucleotide synthesizers. A double-stranded fragment may be obtained from the single stranded product of chemical synthesis either by synthesizing the complementary strand and annealing the strand together under appropriate conditions or by adding the complementary strand using DNA polymerase with an appropriate primer sequence.

DNA constructs prepared for introduction into a prokaryotic or eukaryotic host will typically comprise a replication system (i.e. vector) recognized by the host, including the intended DNA fragment encoding the desired polypeptide, and will preferably also include transcription and translational initiation regulatory sequences operably linked to the polypeptide-encoding segment. Expression systems (expression vectors) may include, for example, an origin of replication or autonomously replicating sequence (ARS) and expression control sequences, a promoter, an enhancer and necessary processing information sites, such as ribosome-binding sites, RNA splice sites, polyadenylation sites, transcriptional terminator sequences, and mRNA stabilizing sequences. Signal peptides may also be included where appropriate from secreted polypeptides of the same or related species, which allow the protein to cross and/or lodge in cell membranes or be secreted from the cell.

Expression and cloning vectors will likely contain a selectable marker, that is, a gene encoding a protein necessary for the survival or growth of a host cell transformed with the vector. Although such a marker gene may be carried on another polynucleotide sequence co-introduced into the host cell, it is most often contained on the cloning vector. Only those host cells into which the marker gene has been introduced will survive and/or grow under selective conditions. Typically selection genes encode proteins that (a) confer resistance to antibiotics or other toxic substances, e.g., ampicillin, neomycin, methotrexate, etc.; (b) complement auxotrophic deficiencies; or (c) supply critical nutrients not available from complex media. The choice of the proper selectable marker will depend on the host cell; appropriate markers for different hosts are known in the art.

It will be recognized by those skilled in the art that the DNA sequences may vary due to the degeneracy of the genetic code and codon usage. All DNA sequences which code for the APN are included in this invention, including DNA sequences as given in SEQ ID NO: 2 having an ATG preceding the coding region for the mature protein.

Additionally, it will be recognized by those skilled in the art that allelic variations may occur in the DNA sequences which will not significantly change activity of the amino acid sequences of the peptides which the DNA sequences encode. All such equivalent DNA sequences are included within the scope of this invention and the definition of the regulated promoter region. The skilled artisan will understand that the sequence of the exemplified APN sequence can be used to identify and isolate additional, nonexemplified nucleotide sequences which will encode functional equivalents to the sequences given in SEQ ID NO: 1, or an amino acid sequence of greater than 90% identity thereto and having equivalent biological activity. DNA sequences having at least 90, or at least 95% identity to the recited DNA sequences of SEQ ID NO: 2 and encoding functioning APN are considered equivalent to the sequences of SEQ ID NO: 2 and are included in the definition of an APN encoding sequence. Following the teachings herein and using knowledge and techniques well known in the art, the skilled worker will be able to make a large number of operative embodiments having equivalent DNA sequences to those listed herein without the expense of undue experimentation.

As used herein percent sequence identity of two nucleic acids is determined using the algorithm of Karlin and Altschul (1990) Proc. Natl. Acad. Sci. USA 87:2264–2268, modified as in Karlin and Altschul (1993) Proc. Natl. Acad. Sci. USA 90:5873–5877. Such an algorithm is incorporated into the NBLAST and XBLAST programs of Altschul et al. (1990) J. Mol. Biol. 215:402–410. BLAST nucleotide searches are performed with the NBLAST program, score=100, wordlength=12, to obtain nucleotide sequences with the desired percent sequence identity. To obtain gapped alignments for comparison purposes, Gapped BLAST is used as described in Altschul et al. (1997) Nucl. Acids. Res. 25:3389–3402. When utilizing BLAST and Gapped BLAST programs, the default parameters of the respective programs (NBLAST and XBLAST) are used. See the government (“.gov”) website (www.) for the National Center for Biotechnology Information/National Institute of Health (“ncbi.nih”).

Various degrees of stringency of hybridization can be employed. The more stringent the conditions, the greater the complementarity that is required for duplex formation. Stringency can be controlled by temperature, probe concentration, probe length, ionic strength, time, and the like. Preferably, hybridization is conducted under moderate to high stringency conditions by techniques well known in the art, as described, for example, in Keller, G. H., M. M. Manak (1987) DNA Probes, Stockton Press, New York, N.Y., pp. 169–170.

As used herein “moderate to high stringency” conditions for hybridization refers to conditions that achieve the same, or about the same, degree of specificity of hybridization as the conditions “as described herein.” Examples of moderate to high stringency conditions are provided herein. Specifically, hybridization of immobilized DNA on Southern blots with 32P-labeled gene-specific probes was performed using standard methods (Maniatis et al.). In general, hybridization and subsequent washes were carried out under moderate to high stringency conditions that allowed for detection of target sequences with homology to sequences exemplified herein. For double-stranded DNA gene probes, hybridization was carried out overnight at 20–25° C. below the melting temperature (Tm) of the DNA hybrid in 6×SSPE, 5× Denhardt's solution, 0.1% SDS, 0.1 mg/ml denatured DNA. The melting temperature is described by the following formula from Beltz et al. (1983).

-   -   Tm=81.5° C.+16.6 Log [Na+]+0.41(% G+C)−0.61 (% formamide)         600/length of duplex in base pairs.

Washes are typically carried out as follows:

-   -   (1) Twice at room temperature for 15 minutes in 1×SSPE, 0.1% SDS         (low stringency wash).     -   (2) Once at Tm−20° C. for 15 minutes in 0.2×SSPE, 0.1% SDS         (moderate stringency wash).

For oligonucleotide probes, hybridization was carried out overnight at 10–20° C. below the melting temperature (Tm) of the hybrid in 6×SSPE, 5× Denhardt's solution, 0.1% SDS, 0.1 mg/ml denatured DNA. Tm for oligonucleotide probes was determined by the following formula from Suggs et al. (1981):

-   -   Tm (° C.)=2 (number T/A base pairs)+4(number G/C base pairs)

Washes were typically carried out as follows:

-   -   (1) Twice at room temperature for 15 minutes 1×SSPE, 0.1% SDS         (low stringency wash).     -   (2) Once at the hybridization temperature for 15 minutes in         1×SSPE, 0.1% SDS (moderate stringency wash)

In general, salt and/or temperature can be altered to change stringency. With a labeled DNA fragment of greater than about 70 or so bases in length, the following can be used:

Low: 1 or 2X SSPE, room temperature Low: 1 or 2X SSPE, 42° C. Moderate: 0.2X or 1X SSPE, 65° C. High: 0.1X SSPE, 65° C.

Duplex formation and stability depend on substantial complementarity between the two strands of a hybrid, and, as noted above, a certain degree of mismatch can be tolerated. Therefore, polynucleotide sequences of the subject invention include mutations (both single and multiple), deletions, and insertions in the described sequences, and combinations thereof, wherein said mutations, insertions, and deletions permit formation of stable hybrids with a target polynucleotide of interest. Mutations, insertions, and deletions can be produced in a given polynucleotide sequence using standard methods known in the art. Other methods may become known in the future.

The mutational, insertional, and deletional variants of the polynucleotide sequences of the invention can be used in the same manner as the exemplified polynucleotide sequences so long as the variants have substantial sequence similarity with the original sequence. As used herein, substantial sequence similarity refers to the extent of nucleotide similarity that is sufficient to enable the variant polynucleotide to function in the same capacity as the original sequence. Preferably, this similarity is greater than 50%; more preferably, this similarity is greater than 75%; and most preferably, this similarity is greater than 90%. The degree of similarity needed for the variant to function in its intended capacity will depend upon the intended use of the sequence. It is well within the skill of a person trained in this art to make mutational, insertional, and deletional mutations that are designed to improve the function of the sequence or otherwise provide a methodological advantage.

In a further embodiment, the subject invention provides expression vectors comprising one or more polynucleotides comprising nucleotide sequences encoding APN and capable of expressing APN in a suitable host cell. In the vectors of the subject invention, the polynucleotide encoding APN is operably linked to suitable transcriptional and/or translational regulatory elements to effect expression of the APN in a suitable host cell. The regulatory elements may be derived from mammalian, microbial, viral or insect genes and include, for example, promoters, enhancers, transcription and translation initiation sequences, termination sequences, origins of replication, and leader and transport sequences. Suitable regulatory elements are selected for optimal expression in a desired host cell.

Possible regulatory sequences can include, but are not limited to, any promoter already shown to be constitutive for expression, such as those of viral origin (e.g., IE1 promoter from Baculoviruses) or so-called “housekeeping” genes (ubiquitin, actin, tubulin) with their corresponding termination/poly A+sequences. In addition, the gene can be placed under the regulation of inducible promoters and their termination sequences so that gene expression is induced by light (rbcS-3A, cab-1), heat (hsp gene promoters) or wounding (mannopine, HGPGs). Other suitable promoters include the metallothionein promoter, dexamethasone promoter, alcohol dehydrogenase promoter, and the baculovirus promoters, i.e., the early promoter (e.g., IE-1 and et1), the late promoters (e.g., vp39 and p6.9), the very late promoters (e.g., po1h and p10) and the hybrid promoter (e.g., vp39/po1h).

It is clear to one skilled in the art that a promoter may be used either in native or truncated form, and may be paired with its own or a heterologous termination/polyA+sequence. In a preferred embodiment, the subject vectors are regulated by D. melanogaster HSP70 promoter.

Expression vectors can be constructed by well known molecular biological methods as described for example in Sambrook et al. (1989), or any of a myriad of laboratory manuals on recombinant DNA technology that are widely available. Expression vectors into which the polynucleotides of the present invention can be cloned under the control of a suitable promoter are also commercially available. Recombinant viral vectors, including retroviral, baculoviral, parvoviral and densoviral vectors can be used but are not particularly preferred. In host cells containing vectors having an inducible promoter controlling the expression of the nucleic acid encoding APN, expression is induced by methods known in the art and suitable for the selected promoter. For example, expression of nucleic acids under the control of the metallothionein promoter is induced by adding cadmium chloride or copper sulfate to the growth media of host cells.

In a specific embodiment, the subject invention provides a host cell containing a vector comprising nucleotide sequences encoding APN under the control of a promoter. The host cell may be procaryotic or eukaryotic, including bacterial, yeast, insect and mammalian cells. Insect and mammalian cells are preferred. Particularly preferred host cells include insect cell lines, such as, for example, Spodoptera frugiperda (Sf9 and Sf21) and Trichoplusia ni (Tn cells), Estigma acrae (Ea4 cells), Drosophila melanogaster (Dm cells), Choristoneura fumiferama (Cf-y cells), Mamestra brassicae (MaBr-3 cells), Bombyx mori (MnN-4 cells), Helicoverpa zea (Hzlb3 cells), and Lymantria dispar (Ld652Y cells), among others. The host cells may be transformed, transfected or infected with the expression vectors of the present invention by methods well-known to those of ordinary skill in the art. Transfection may be accomplished by known methods, such as liposome mediated transfection, calcium phosphate mediated transfection, microinjection and electroporation.

Cells of the subject invention may be transfected with a polynucleotide comprising a nucleotide sequence of FIGS. 2A–2E (SEQ ID NO:2), or fragment thereof. One skilled in the art would readily appreciate that polynucleotides encoding other APNs may be substituted for FIGS. 2A–2E (SEQ ID NO:2). Examples of toxin binding APNs have been reported for several species (see, e.g., Knight et al., 1994; Sangadala et al., 1994; Gill et al., 1995; Luo et al., 1996; Yaoi et al., 1997; Denolf et al., 1997; and Huo et al., 1998 incorporated herein by this reference). Equipped with the teachings herein, the skilled artisan would be able to transfect insect cells with these, as well as future isolated APN-encoding polynucleotides, to produce APN expressing cells.

The skilled artisan will note that polynucleotides preferred for practicing the subject invention encode proteins capable of expression in cells, localization to cell membrane, and toxin binding. Accordingly, fragments of APN sequences as well as functional mutants may equally be used in practicing the subject invention. Such fragments and mutants will be readily obtainable following the teachings herein coupled with the state of the art. For example, using specifically exemplified polynucleotides as probes, useful polynucleotides can be obtained under conditions of appropriate stringency. Standard hybridization conditions include hybridization with nonspecific DNA, such as salmon DNA, at 50° C. and washing at 45° C. To obtain polynucleotides having the lowest detectable homology with exemplified APNs, hybridization is conducted under conditions of low standard stringency (30–37° C. and 4–6×SSC). More closely related APN-like polynucleotides can be obtained under moderate standard stringency conditions (40–50° C. in 1×SSC).

In a further embodiment, the subject invention is directed to a method of identifying novel pesticide agents comprising the steps of obtaining cells transfected with one or more of the polynucleotides encoding APNs, whereby said polynucleotides are expressed to produce at least one protein that is localized at the cell membrane of said cells; and screening one or more pesticide agents for their ability to produce an observable effect on said cells. The observable effect may be related to a change in metabolism or morphology. The effect may be cytotoxic which may manifest itself, for example, as reduced thymidine uptake, slower increase in optical density of the culture, reduced exclusion of vital dyes (e.g., trypan blue), increased release of viability markers such as chromium and rubidium and the like. The differential response between the pesticide-treated cells and the cells absent the pesticide may be qualitatively or quantitatively noted. Further, the strength of the pesticide can be assessed by noting the strength of the response. While the subject invention is useful for screening a variety of pesticide agents, one skilled in the art will appreciate that the subject methods are particularly useful in identifying novel natural or mutated B.t. toxins.

In a further embodiment, the subject invention is directed to novel pesticide agents obtained by the subject screening methods.

In yet another embodiment, the subject invention is drawn to a method of identifying novel APN inhibitors comprising obtaining cells as described herein, and screening compounds of interest for their ability to inhibit aminopeptidase activity.

The teachings of all patents and publications cited throughout this specification are incorporated by reference in their entirety to the extent not inconsistent with the teachings herein.

Following are examples which illustrate procedures for practicing the invention. These examples should not be construed as limiting. All percentages are by weight and all solvent mixture proportions are by volume unless otherwise noted.

EXAMPLE 1

1.1. Cloning and Sequencing of M. sexta 120 kDa APN

A PCR strategy similar to that described in Tresnan et al. (1996) was used to clone the 120 kDa APN1 a cDNA from M. sexta. Four PCR primers were designed from the published 120 kDa APN1 sequence (Knight et al., 1995): MS1, 5′ATTTTCTTGGGGGTCGCCCTTC3′ (SEQ ID NO: 3); MS1R, 5′ACGCTACCATGTTAATG3′ (SEQ ID NO:4); MS5R, 5′TGCTGTGTCATTCTGAG3′ (SEQ ID NO: 5); MS4, 5′ AGGAGATTCGCCCATGACGCC3′ (SEQ ID NO: 6). Two vector primers were T3,5′AATTAACCCTCACTAAAGGG3′ (SEQ ID NO: 7) and T7, 5′GTAATACGACT CACTATAGGGC3′ (SEQ ID NO: 8). Primers were synthesized by the Molecular Genetics Facility (University of Georgia). A midgut cDNA library from M. sexta constructed in ZAPII vector (Stratagene, La Jolla, Calif.) was kindly provided by Dr. R. Graf. (Zoologisches Institut der Universitat, Munich, Germany).

All PCR reagents were purchased from FisherBiotech (Pittsburgh, Pa.). PCR was performed as follows: 30 cycles of 55° C./2 min, 72° C./3 min, and 94° C./1 min. A sample of the phage stock from cDNA library (5 μl, titer: 1×10⁶ pfu/μl) was heated at 100° C. for 5 min, and then used as a template. Primers MS1 and MS1R, corresponding to sequences of the 5′ and 3′ ends of the APN cDNA amplified a 3 kb product. Primers MS5R and T3 gave a 1.4 kb PCR product. The 3 kb and 1.4 kb PCR products were each cloned into pGEM-T (Promega, Madison, Wis.) and called pGEM6 and pGEM4.5, respectively. The plasmid p120 was constructed by the ligation of a 0.4 kb NotI/EcoRV fragment of pGEM4.5 into the same sites of pGEM6. The third primer pair, MS4 and T7, was used to amplify the 3′ end of APN from the cDNA library. The resulting 2 kb PCR product was digested with Bg1II and ApaI and separated on a low-melting temperature agarose gel. The desired fragment of about 0.4 kb was purified and cloned into the Bg1II/ApaI sites of p120 to give pAPN120. The 3.2 kb insert of pAPN120 was completely sequenced in both strands using a ALF DNA Sequencer (Molecular Genetics Facility, University of Georgia).

1.2 Construction of Expression Plasmids, pET120-2, pET120-3 and pET1000.

The plasmid pET30A (Novagen, Madison, Wis.) was used to express recombinant APN in E. coli. For construction of pET120-2, the entire coding region of 120 kDa APN cDNA of M. sexta (3.2 kb) was excised from pAPN120 with BamHI and XhoI, and then purified and cloned into pET30A digested with the same enzymes. The plasmid pET120-3 was constructed by inserting an EcoRV-XhoI fragment of pAPN120 into pET30A. The EcoRV-XhoI fragment of 120 kDa APN cDNA encodes a 5′-truncated 96 kDa protein. The plasmid pET 1000 carrying the 5′ and 3′-truncated form of APN (31 kDa) was constructed by the ligation of a 851 bp EcoRV-SacI fragment of 120 kDa APN cDNA into pET30A. These constructions were verified by restriction enzyme analyses or DNA sequencing.

1.3 In Vitro Transcription and Translation

In vitro transcription and translation were performed using the Single Tube Protein System 2 (STP2) (Novagen, Madison, Wis.) according to the manufacturer's instructions. Plasmids, pET120-2 and pET120-3, were transcribed at 30° C. for 15 min and then translated by adding 30 μl of STP2 translation mix and 4 μl of ³⁵S-methionine. After incubation at 30° C. for 60 min, the reaction mixture (5 μl) was treated with 20 μl of Laemmli sample buffer. Laemmli (1970). The sample was heated at 90° C. for 5 min and then centrifuged at 12 000×g for 2 min. The supernatant was separated by SDS-10% polyacrylamide gel electrophoresis (SDS-PAGE). After electrophoresis, the gel was immersed in staining buffer for 10 min, rinsed briefly with water, and exposed to X-ray film at room temperature for 12 h.

Results

A M. sexta midgut cDNA library in λZAPII vector was used as a template for PCR amplification of full-length or partial cDNAs for the 120 kDa APN. Primers (MS1/MS2) corresponding to sequences from the 5′ and 3′ ends of the cloned M. sexta 120 kDa APN1 cDNA (Knight et al, 1995) gave the expected 3 kb product. A second primer (MS5) and vector primer (T3) amplified a 1.4 kb product, while the third pair of primers, MS4 and T7, produced a 2 kb fragment. DNA sequencing showed that the 5′ and 3′ ends of the 3 kb and 2 kb PCR fragments were identical to the published sequence of M. sexta 120 kDa APN1 (Knight et al., 1995). The 3′ end of the 1.4 kb fragment had the expected APN nucleotide sequence, however the 5′ end had an additional 30 nucleotides. A full-length cDNA clone, called pAPN120, was constructed from the overlapping PCR clones and completely sequenced from both DNA strands (FIGS. 2A–2E). Plasmid pAPN120 contains a 3164 bp insert with a 2985 bp open reading frame beginning with a consensus Kozak sequence (AGAATGG) at nucleotide 15. Kozak (1987). The open reading frame encodes a protein of 995 amino acids including additional 5 amino acids upstream of the reported M. sexta APN1 clone. Knight et al. (1995). A comparison of the amino acid sequences of our APN cDNA clone (APN1a) and APN1 showed that our clone has 8 amino acids different from the APN1 (FIG. 3). These different amino acids do not modify any putative glycosylation or GPI-anchor sites. Interestingly, some of these amino acids (for example, L²¹¹, I³¹³, Y⁴²², T⁵⁶⁸, E¹⁰⁰⁷) in our APN1a clone (SEQ ID NO:1) are actually identical to, or share electrostatic properties or polarity with other cloned APNs FIG. 3 (SEQ ID NOS:9, 10, 1, 11, 12, and 13) disclosed, for example, in Knight et al. (1995) (SEQ ID NO:11); Gill et al. (1995) (SEQ ID NO: 13); Denolf et al. (1997) (SEQ ID NOS. 9 and 10); and Hua et al. (1998) (SEQ ID NO: 12). In vitro transcription and translation of APN1a cDNA in a rabbit reticulocyte lysate system was done to establish the molecular size of non-glycosylated APN1a. Plasmid pET120-2 carrying full-length APN1a cDNA resulted in a protein of 110 kDa (FIG. 5, lane 3), while pET120-3 containing 5′-truncated APN1a cDNA gave a 96 kDa protein (FIG. 5, lane 4). The control plasmid, pCITEβ-gal, containing the E. coli β-galactosidase gene yielded a 116 kDa protein (FIG. 5, lane 2). The molecular size of translated APN1a was the same as predicted from the deduced amino acid sequence, supporting the putative translation start site in our cDNA clone. Also, the determined 110 kDa size for non-glycosylated APN1a provided a reference size standard for comparison with APN in M. sexta BBMV and cultured insect cells.

EXAMPLE 2

2.1 Expression of the Recombinant APN in E. coli and Production of the Polyclonal Antibody

To express truncated 31 kDa APN in E. coli, plasmid pET1000 was electroporated into an E. coli expression host, BL21(DE3), and recombinant HIS-tagged APN expressed and purified according to the manufacturer's instructions (Novagen, Madison, Wis.) using a Histrap column (Pharmacia, Piscataway, N.J.). Eluted fractions containing expressed APN were pooled and successively dialyzed against 4 M, 2 M, 1 M and 0 M urea in 50 mM Na₂CO₃, pH 10. The resulting sample was then centrifuged at 27 000×g for 30 min. The pellet was suspended in 1 ml of PBS (phosphate-buffered saline), and the supernatant was concentrated to 1 ml. Protein concentration in the supernatant and pellet were about 0.5 and 0.8 mg/ml, respectively. Both samples were stored at −80° C. until use. Antisera against 31 kDa truncated APN was raised by immunization of a NZW (SPF) rabbit with 0.2 mg of truncated APN administered in complete Freud's adjuvants (Polyclonal Antibody Production Service, University of Georgia). The rabbit was boosted two times with truncated APN protein. The rabbit serum was collected 10 days after the second boost. Reactivity of the rabbit serum was assessed by Western blotting. IgG was purified using a ProteinA column and kit (Pierce, Rockford, Ill.).

2.2 Construction of the Expression Vector pHSP120

The plasmid pHSP70PL that contains the Drosophila melanogaster HSP70 promoter and the 5′ untranslated leader of HSP70 is described in Morris and Miller (1992). We first constructed plasmid pHSP-HR5. This plasmid contains the polyadenylation sequence (poly A) from the p35 gene and half of the homologous region 5 (hr5) of Autographa californica nuclear polyhedrosis virus (AcMNPV), a sequence extending from nucleotide 17,344 to 17,636 (sequence according to Ayres et al., 1994). Two PCR primers (5′-pAHR5: 5′GGAAGATCTTCCACTGCATGCGTAACTAGTGC ACTCAAC3′(SEQ ID NO: 14) and 3′-pAHR5: 5′GGGATCCCGTCCCCGCGGGGACTCGATTT GAAAAACAAATGACCATCATC3′ (SEQ ID NO: 15)) were designed to amplify the poly A and a part of the hr5 sequence from the plasmid pH1PQ which contains the Hind III Q restriction fragment of AcMNPV genomic DNA. The PCR product (316 bp) was digested with Bg1II and BamHI, and then inserted into pHSP70PL vector treated with Bg1II. The resulting plasmid, pHSP-HR5, was verified by restriction enzyme analyses and DNA sequencing.

Plasmid pHSP120 was then constructed by inserting a 3.2 kb SphI/BamHI fragment of pAPN120 into pHSP-HR5. The detailed structure of pHSP120 is shown in FIG. 1.

2.3 Transfection and Expression of the 120 kDa APN in Sf21 Cells

The pHSP120 plasmid DNA was isolated and purified using a Plasmid Maxi Kit (Qiagen, Valencia, Calif.). Sf21 (IPLB-Sf21) cells (Vaughn et al., 1977) were plated at 1.8×10⁶ cells per plate (60 mm diameter) prior to transfection with pHSP120 DNA. Plasmid DNA (10 μg) was mixed with 5 μl of Lipofectin reagent (Gibco BRL, Gaithersburg, Md.) and combined with 1 ml of TC-100 medium (Gibco BRL, Gaithersburg, Md.). The mixture was incubated at room temperature for 15 min. Following 3 washes with TC-100 medium, the DNA/Lipofectin mixture was added to the cells, and then the cells were incubated at room temperature for 4 h on a rocker. After removal of DNA/Lipofectin mixture, TC-100 with 10% fetal bovine serum (4 ml) was added to the plates and the cells were incubated at 27° C. overnight. Expression of 120 kDa APN in Sf21 cells was induced by heat-shocking cells at 42° C. for 30 min the following day. After incubation at 27° C. for 24 h, the cells were collected and analyzed using immunofluorescence localization and Western blotting.

2.4 Immunoblot Analysis

Non-heat-shocked or heat-shocked insect cells (10⁷ cells) were lysed in 200 μl Laemmli sample buffer (Laemmli, 1970), and the sample was centrifuged at 12 000×g for 5 min. The supernatant (15 μl) was separated by 8% SDS-PAGE and electrophoretically transferred to nitrocellulose membrane (Millipore Corp., Bedford, Mass.) in transfer buffer (Towbin et al., 1979) at 4° C. overnight. The membrane was blocked with 5% non-fat dry milk in PBST at room temperature for 1 h, and then probed with anti-APN antibody (0.5 μg/ml) in PBST containing 0.1% non-fat dry milk for 2 h. After three washes with PBST, the membrane was incubated with horseradish peroxidase (HRP)-conjugated donkey anti-rabbit IgG (1:3000) (Amersham, Piscataway, N.J.) in PBST containing 0.1% non-fat dry milk at room temperature for 2 h. Protein bands were visualized with ECL Western Blotting Kit (Amersham, Piscataway, N.J.) according to manufacturer's instructions.

Results

The heat-shock expression vector pHSP120 containing M. sexta APN 1 a cDNA (FIG. 1) was transfected into Sf21 cells, following which the cells were heat-shocked and tested for the presence of APN. As shown in FIG. 4, anti-APN antibody specifically recognized the expressed APN (FIG. 4, lane 2), but not the proteins from Sf21 cells transfected with control vector (FIG. 4, lane 1). This anti-APN antibody also bound to 115 kDa APN purified from M. sexta midgut BBMV (FIG. 4, lane 3). The 115 kDa APN is the PIPLC-cleaved form of 120 kDa APN. Lu and Adang (1996). FIG. 6A shows a Western blot of Sf21 cells transfected with increasing amounts of plasmid pHSP120 DNA. Optimal expression of 120 kDa APN1a was achieved when 10 μg of DNA was used to transfect Sf21 cells (FIG. 6A). Optimal times for detecting expressed 120-kDa APN1a in Sf21 cells were between 12 h and 24 h after heat-shock (FIG. 6B). The molecular size of expressed APN1a was estimated to be 120 kDa, which is the same as that from M. sexta BBMV (FIG. 6A). Several closely migrating bands are visible in FIGS. 6A and B, suggesting that M. sexta APN1a may be heterogeneously glycosylated in Sf21 cells.

EXAMPLE 3

3.1 PIPLC Digestion and GPI Anchor Detection

Sf21 cells (10⁷ cells) were lysed in 200 μl Laemmli sample buffer (Laemmli, 1970), and 15 μl of supernatant prepared as described above, was separated by SDS-PAGE, and then electrophoretically transferred to nitrocellulose membrane (Millipore Corp., Bedfore, Mass.). The membrane was blocked with 50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 1 mM EDTA, 0.05% Tween-20 (buffer A) containing 3% BSA at room temperature for 2 h. After three washes with buffer A, the membrane was treated with phosphatidylinositol-specific phospholipase C (PIPLC, Sigma) (1.5 U) overnight at room temperature in 10 ml of 20 mM Tris-HCl, pH 7.4, 0.1% Triton-X100, 1 mM DTT and 3% BSA. The presence of a cleaved GPI group was detected using a polyclonal antibody against the cross-reacting determinant (CRD) of GPI-anchored proteins as described previously (Garczynski and Adang, 1995) (anti-CRD serum was kindly provided by Dr. K. Mensa-Wilmot, University of Georgia). Protein bands were visualized with ECL Western blotting Kit (Amersham, Piscataway, N.J.).

3.2 Preparation of BBMV and Purification of M. sexta 115 kDa APN

M. sexta larvae were reared on artificial diet (Southland Products, Inc.; Lake Village, Ark.) at 26° C., 70% relative humidity with a photoperiod of 12:12 (Light:Dark) h. Midguts were dissected from second day 5th instar larvae, and either immediately used to prepare brush border membrane vesicles (BBMV) or stored at −80° C. BBMV were prepared according to that described in Wolfersberger et al. (1987) as modified in Ferre et al. (1991) and stored in 0.3 M mannitol, 5 mM EGTA, 17 mM Tris-Cl, pH 7.5, at −80° C. until needed. The 115 kDa M. sexta APN was purified as described previously (Lu and Adang, 1996).

3.3 Immunofluorescence Localization

Sf21 cells were plated onto a microscope cover glass (18×18 mm) in a tissue culture dish (60×15 mm diameter), and then transfected and heat-shocked as described above. After three washes with Insect PBS (1 mM Na₂HPO₄, 10.5 mM KH₂PO₄, 140 mM NaCl, 40 mM KCl, pH 6.2), the cells were fixed in 1 ml of ice-cold methanol for 5 min. The fixed cells were washed three times with standard PBST (phosphate-buffered saline containing 0.1% Tween 20) and blocked with 5% non-fat dry milk in PBST at 4° C. overnight. Cells were then incubated with anti-APN IgG (1 μg/ml) in PBST containing 0.1% dry milk at room temperature for 1 h. After incubation cells were washed three times, then incubated with Alexa-conjugated goat anti-rabbit IgG (Molecular Probes, Eugene, Oreg.) (1:1000) in PBST containing 0.1% dry milk for 1 hour at room temperature. Immunostained cells were observed using fluorescence microscopy or fluorescence confocal microscopy (Center for Advanced Ultrastructural Research, University of Georgia).

3.4 Assays of Aminopeptidase N Activity

Sf21 cells transfected with pHSP120 or pHSP-HR5 were collected and washed two times with PBS. The cells were suspended in 2 ml PBS and then homogenized with Potter-Helvehjem homogenizer. The cell homogenate was centrifuged at 27,000×g for 15 min at 4° C. The pellet was re-suspended in 2 ml of 10 mM Tris-HCl, 150 mM NaCl, pH 7.4. APN activity was assayed with L-leucine-p-nitroanilide as substrate as described previously (4). Briefly, the APN substrate (50 μl 6.8 mg/ml in methanol) was mixed with 1.4 ml of substrate buffer (10 mM Tris-HCl, 150 mM NaCl pH 7.4), and then the homogenate (50 μg of proteins) was added. The sample solution was incubated at 37° C. for 2 h. The level of APN activity was then determined by spectrophotometric measurement of free p-nitroanilide at 405 nM.

Results

M. sexta 120 kDa APN is attached to the epithelial membrane of midgut cells by a GPI anchor (Garczynski and Adang, 1995; Lu and Adang, 1996). Both 120 kDa APN1a and M. sexta APN1 have a putative GPI signal sequence at the C-terminus (FIG. 3; Knight et al., 1995) (SEQ ID NO:11). We examined the GPI-anchor properties of the expressed 120 kDa APN1a in Sf21 cells using anti-CRD antibody that is specific for the modified inositol product resulting from PIPLC cleavage. As shown in FIGS. 7A and B, expressed APN1a reacted with the anti-CRD antibody after PIPLC treatment (FIG. 7A, lane 2). Without PIPLC treatment, the anti-CRD antibody did not recognize expressed APN1a (FIG. 7B, 2). These results indicate that the expressed APN1a has an intact GPI anchor. Also, the anti-CRD antibody clearly recognized 115 kDa and 120 kDa protein bands (FIG. 7A, lane 2). These two processed forms of 120 kDa APN1a are likely due to differences in glycosylation. FIGS. 7A and B also revealed that the anti-CRD antibody did not bind to PIPLC-treated proteins in Sf21 cells not expressing APN1a (FIG. 7A, lane 1), thus indicating that Sf21 cells appear not to express any endogenous GPI-linked proteins.

Since 120 kDa APN1a was expressed by transfected Sf21 cells as a GPI-linked protein, we investigated if APN1a was on the cell membrane surface by immunofluorescence microscopy. Sf21 cells on glass coverslips were incubated with anti-APN antibody and Alexa488-anti-IgG conjugate. Cells were viewed by fluorescence confocal microscopy. The results are shown in FIGS. 8A–8D. An intense fluorescent signal specific for anti-APN antibody was observed on some Sf21 cells (FIG. 8C), indicating that expressed APN1a was located primarily on the extracellular membrane. Sf21 cells transfected with control vector, pHSP-HR5, showed no significant immunofluorescence (FIG. 8D). Also, Sf21 cells stained with pre-immune serum did not show any fluorescent signal (FIGS. 8A and B).

APN Activity Assays of the Expressed M. sexta APN1a

Sf21 cells transfected with pHSP120 or pHSP-HR5 were collected and homogenized. The homogenate (50 μg of proteins) was tested for APN activity. The results were shown in FIG. 9. A high level of APN activity was present in the cells transfected pHSP120 compared with that in the cells transfected with pHSP-HR5 (FIG. 9, columns A and C). Amastatin, an APN inhibitor, greatly reduced APN activity of pHSP120-transfected Sf21 cells (FIG. 9, column B). In contrast, amastatin did not show significantly impact on enzymatic activity of Sf21 cells transfected with pHSP-HR5 (FIG. 9, column D).

EXAMPLE 4

4.1 Purification and Labeling of B. thuringiensis Toxins

Growth of B. thuringiensis strains, trypsin activation, and fast-performance liquid chromatography (Pharmacia) purification of Cry1Ac, Cry1Ca and Cry3a toxins were done as previously described (Garczynski et al 1991; Lu and Adang 1996). Two forms (55 kDa and 65 kDa) of Cry1 Ba toxins were kindly provided by Dr. Masson (Biotechnology Research Institute, National Research Council of Canada, Canada). The purified Cry1Ac and Cry1Ba (200 μg) were labeled with 5-(and-6)-carboxytetramethylrhodamine, succinimidyl ester [5(6)-TAMRA SE] (Molecular Probe, Eugene, Oreg.) according to the manufacturer's instructions. Briefly, 5(6)-TAMRA (5 mg) were dissolved in 0.5 ml of Dimethylformamide (DMF) immediately before starting the labeling. The dye solution (100 μl) was slowly added into 500 μl of toxin sample (200 μg toxins) and incubated for 1 h at room temperature. The reaction was stopped by adding 0.1 ml of 1.5 M hydroxylamine, pH 8.5, and then the mixture was incubated for 1 h at room temperature. The sample was applied onto a 25 ml of G-50 gel filtration column (1.5×25 cm) equilibrated with 20 mM Na₂CO₃, pH 9.6. The fractions containing toxins were pooled and stored at −80° C. until use.

4.2 Binding of B. thuringiensis Toxins and Anti-APN Serum to Sf21 Cells

Sf21 cells were plated onto a microscope cover glass (18×18 mm) in a tissue culture dish (60×15 mm diameter), and then transfected and heat-shocked as described above. After three washes with Insect PBS (1 mM Na₂HPO₄, 10.5 mM KH₂PO₄, 140 mM NaCl, 40 mM KCl, pH 6.2), the cells were fixed in 1 ml of ice-cold methanol for 5 min. The fixed cells were washed three times with standard PBST (phosphate-buffered saline containing 0.1% Tween 20) and blocked with 5% non-fat dry milk in PBST at 4° C. overnight. Cells were then incubated with either rhodamine-labeled B. thuringiensis toxins (1 μg/ml) or anti-APN IgG (1 μg/ml) in PBST containing 0.1% dry milk at room temperature for 1 h. The incubation cells were washed three times with PBST. And then the cells treated by anti-APN antibody were further incubated with Alexa488-conjugated goat anti-rabbit IgG (Molecular Probes, Eugene, Oreg.) (1:1000) in PBST containing 0.1% dry milk for 1 hour at room temperature. Immunostained cells were observed using fluorescence microscopy or fluorescence confocal microscopy (Center for Advanced Ultrastructural Research, University of Georgia).

4.3 Affinity Purification and Immunoblot Analyses of 120 kDa APN

Affinity purification method of M. sexta 120 kDa APN from Sf21 cells or M. sexta midgut BBMV was described in Luo et al (1996). Briefly, Cry1Ac, Cry1Ba and Cry3a toxins (200 μg) were coupled to 400 μl of cyanogen bromide-activated Sepharose 4B in 20 mM Na₂CO₃, pH 9.6. M. sexta BBMV (1 ml, 3.4 mg/ml proteins) or Sf21 cells (10⁷ cells) were solubilized with 2% 3-[(3-cholamidopropyl) dimethylammonio]-1-propane-sulphonate (CHAPS) in buffer A (50 mM Na₂CO₃, pH 9.6, 200 mM NaCl, 5 mM EGTA, 0.1% CHAPS). Insoluble material was removed by centrifugation at 27,000×g for 30 min at 4° C. CHAPS-solubilized BBMV (1 ml) or Sf21 cells (2 ml) were then added to the toxin-coupled Sepharose beads equilibrated with Buffer A. The mixture was incubated at 4° C. overnight. After washing with 100 ml of Buffer A, the binding protein-toxin complex was dissociated from the beads by heating in SDS-PAGE sample buffer at 100° C. for 5 min. The samples were separated by 8% SDS-PAGE, and then electrophoretically transferred to nitrocellulose membrane (Millipore Corp., Bedford, Mass.) in transfer buffer (Towbin et al., 1979) at 4° C. overnight. The membrane was blocked with 5% non-fat dry milk in PBST at room temperature for 1 h, and then probed with anti-APN antibody (0.5 μg/ml) in PBST containing 0.1% non-fat dry milk for 2 h. After three washes with PBST, the membrane was incubated with horseradish peroxidase (HRP)-conjugated donkey anti-rabbit IgG (1:3000) (Amersham, Piscataway, N.J.) in PBST containing 0.1% non-fat dry milk at room temperature for 2 h. Protein bands were visualized with ECL Western Blotting Kit (Amersham, Piscataway, N.J.) according to manufacturer's instructions.

4.4 Toxicity Assays

Sf21 cells (1.8×10⁶) were grown and heat-shocked as described above. Cry1Ac, Cry1Ba and Cry1Ca toxins were diluted to 5, 10, 25, and 40 μg/ml with insect PBS, pH 6.2. The freshly prepared toxin solution (1 ml) was added to each Sf21 cell plate and incubated at room temperature for 2 h. Cell mortality was determined by trypan blue staining as described by Thomas and Ellar (1983).

Results

Previous studies showed that M. sexta 120 kDa APN bound specifically to Cry1Aa, Cry1Ab and Cry1Ac, but not Cry1C or Cry1E toxins (Knight et al., 1994; Sangadala et al., 1994; Masson et al, 1995; Lu and Adang, 1996). Since the expressed 120 kDa APN located on Sf21 cell surfaces, we examined the binding of Cry1 toxins to Sf21 cells using double immunofluorescence staining. Sf21 cells on glass coverslips were incubated with anti-APN antibody and rhodamine-labeled toxins. And then cells were viewed by fluorescence confocal microscopy. The results were shown in FIGS. 10A–D, 11A–D, and 12A–D. No fluorescence was detected in Cry1Ac-staining Sf21 cells (FIGS. 10B and D), suggesting that Cry1Ac does not bind to the expressed APN. The 55 kDa form of Cry1 Ba toxin bound strongly to Sf21 cells transfected with pHSP120 (FIG. 11D), but not to Sf21 cells transfected with pHSP-HR5 (FIG. 10B). However, the 65 kDa form of Cry1 Ba only bound weakly to some Sf21 cells transfected with pHSP120 (FIGS. 12B and D).

B. thuringiensis toxin affinity chromatography was used to purify the expressed M. sexta APN from Sf21 cells. CHAPS-solubilized Sf21 cells were applied to Cry1Ac, Cry1Ba or Cry3a affinity column, respectively. The bound proteins were dissociated from the column, and separated by SDS-PAGE. The proteins were electrophoretically transferred to nitrocellulose membrane and then probed with anti-APN antibody. The results were shown in FIG. 13. The 55 kDa form of Cry1Ba bound strongly to the expressed 120 kDa APN from Sf21 cells (FIG. 13, 1) and native 120 kDa APN from M. sexta BBMV (FIG. 13, lane 5). In contrast, the 65 kDa form of Cry1Ba bound weakly to 120 kDa APN from both Sf21 cells and M. sexta BBMV (FIG. 13, lanes 2 and 6). Cry1Ac and Cry3a toxins did not bind to the expressed 120 kDa APN from Sf21 cells (FIG. 13, lanes 3 and 4).

The toxicity of three Cry1 toxins to Sf21 cells expressing M. sexta 120 APN was determined. A single concentration of Cry1 toxins (40 μg/ml) was used and the cell mortality was measured by Trypan blue staining (Thomas and Ellar 1983). The results were shown in Table 1. As reported previously (Wang and McCarthy 1997), Cry1Ca is highly toxic to Sf21 cells, since more than 85% of the cells were killed 2 h after incubation with Cry1Ca toxin (Table 1). Cry1Ac showed low toxicity to both Sf21 cells transfected with pHSP120 and control plasmid pHSP-HR5, and no significant difference between two treatments was observed (Table 1). The 65 kDa form of Cry1Ba had low toxicity to Sf21 cells. The mortality of cells is about 17% (Table 1). Interestingly, it was found that the 55 kDa form of Cry1Ba is highly toxic to Sf21 cells that expressed M. sexta 120 kDa APN (Table 1). The mortality of Sf21 cells transfected with pHSP120 is 47.3%, while the mortality of the cells transfected with control vector only is 30.0% (Table 1). The concentration dependence of Cry1Ba (55 kDa form) to Sf21 cells transfected with pHSP120 and pHSP-HR5 was further investigated. The results showed that for three toxin concentrations (10, 25, and 40 μg/ml, n=3) significant difference in mortality between the pHSP120-transfected Sf21 cells and pHSP-HR5-transfected cells was observed (FIG. 14). Taken together, these results demonstrate that the 120 kDa APN expressed in Sf21 cells increased the binding and toxicity of 55 kDa form of Cry1Ba to Sf21 cells.

TABLE 1 Toxicity of Three Cry1 Toxins to Sf21 cells Mortality (%) (±SE)^(a) Cells transfected with Cells transfected with Toxin pHSP 120^(b) pHSP-HR5^(c) Cry1Ba (55 kDa) 47.3 (±3.2) 30.0 (±2.3) Cry1Ba (65 kDa) 17.6 (±0.9) 16.5 (±0.1) Cry1Ac 20.0 (±0.4) 20.6 (±0.5) CryaCa 85.7 (±1.1) 84.9 (±0.5) ^(a)Cell mortality was determined by Trypan blue (0.1%) staining 2 h after incubation with 40 μg/ml of Cry1Ba, Cry1Ac, and Cry1Ca toxins in insect PBS. Results are means of three assays (± standard error) ^(b)Plasmid pHSP120 contains a full-length M sexta 120 kDa APN cDNA. ^(c)Plasmid pHSP-HR5 is a control vector without M sexta APN cDNA.

EXAMPLE 5

5.1 Construction of the Expression Vector pHSP120

The plasmid pHSP70PL that contains the Drosophila melanogaster HSP70 promoter and the 5′ untranslated leader of HSP70 is described by Morris and Miller (1992). We first constructed plasmid pHSP-HR5. This plasmid contains the polyadenylation sequence (poly A) from the p35 gene and half of the homologous region 5 (hr5) of Autographa californica nuclear polyhedrosis virus (AcMNPV), a sequence extending from nucleotide 17,344 to 17,636. Two PCR primers (5′-pAHR5: 5′GGAAGATCTTCCACTGCATGCGTAACTAGTGCACTCAAC3′ (SEQ ID NO: 14) and 3′-pAHR5:5′GGGATCCCGTCCCCGCGGGGACTCGATTTGAAAAACAAATGACCATCATC 3′ (SEQ ID NO: 15)) were designed to amplify the poly A and a part of the hr5 sequence from the plasmid pH1PQ which contains the Hind III Q restriction fragment of AcMNPV genomic DNA. The PCR product (316 bp) was digested with BgIII and BamHI, and then inserted into pHSP70PL vector treated with BgIII. The resulting plasmid called pHSP-HR5. Plasmid pHSP120 was then constructed by inserting a 3.2 kb SphI/BamHI fragment of pAPN120 into pHSP-HR5. All plasmids were verified by restriction enzyme analyses and DNA sequencing.

5.2 Construction of pHSPAC120

Plasmid pBSIE1 Gpac, which contains Puromycin acetyltransferase (Pac) gene under IE1 promoter control, was digested with EcoRV and BamHI. A 1.6 kb fragment contained Pac gene and IE1 promoter was purified, and then inserted into pHSP120 treated with the same enzymes. The resulting vector, called pHSPAC120, was verified by restriction enzyme digestion.

5.3 Selection of Stable Sf2 Cell Line

The pHSPAC120 plasmid DNA was isolated and purified using a Plasmid Maxi Kit (Qiagen, Valencia, Calif.). Sf21 cells were plated at 1.8×10⁶ cells per plate (60 mm diameter) prior to transfection with pHSPAC120 DNA. Plasmid DNA (10 μg) was mixed with 5 μl of Lipofectin reagent and combined with 1 ml of TC-100 medium. The mixture was incubated at room temperature for 15 min. Following 3 washes with TC-100 medium, the DNA/Lipofectin mixture was added to the cells, and then the cells were incubated at room temperature for 4 h on a rocker. After removal of DNA/Lipofectin mixture, TC-100 with 10% fetal bovine serum (4 ml) was added to the plates and the cells were incubated at 27° C. overnight. After the medium was removed, the fresh medium (4 ml) containing different concentrations of puromycin (1 μg, 2 μg and 4 μg/ml) was added to different plates respectively. The cells were further incubated at 27° C. for three days. After removal of TC-100 media containing puromycin, 4 ml fresh media were added. The cells were cultured overnight. The alive cells were collected, and the resuspended in TC-100 containing 4 μg/ml puromycin. The cells were plated into a 60 mm plate and incubated for 3 days. The cells were then selected two more times using puromycin as described above.

It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and the scope of the appended claims.

REFERENCES

-   Ayres, M. D., Howard, S. C., Kuzio, J., Lopez-Ferbet, M. and     Dossec, R. D. (1994) “The complete DNA sequence of Autograph     californica nuclear polyhedrosis virus,” Virology 202:586–605. -   Benbacer, L., Kut, E., Besnardeau, L., Laude, H. and     Delmas, B. (1997) “Interspecies aminopeptidase-N chimeras reveal     species-specific receptor recognition by canine coronavirus, feline     infectious peritonitis virus, and transmissible gastroentritis     virus,” J. Virol. 71:734–737. -   Chaudhri, M., Steverding, D., Kittelberger, D., Tjia, S. and     Overath, P. (1994) “Expression of     glycosylphosphadylinositol-anchored Trypanosoma brucei     transferrin-binding protein complex in insect cells,” Proc. Natl.     Acad. Sci. U.S.A. 91:6443–6447. -   Davies, A. and Morgan, B. P. (1993) “Expression of the     glycosylphosphatidlinositol-linked complement-inhibiting protein     CD59 antigen in insect cells using a baculovirus vector,”     Biochemical Journal 295:889–896. -   Delmas, B., Gelfi, J., Haridon, R. L., Vogel, L. K., Sjostrom, H.,     Noren, O. and Laude, H. (1992) “Aminopeptidase N is a major receptor     for the enteropathogenic coronavirus TGEV,” Nature 357:417–419. -   Denolf, P., Hendrickx, K., Van Damme, J., Jansens, S., Peferoen, M.,     Degheele, D. and Van Rie, J. (1997) “Cloning and characterization of     Manduca sexta and Plutella xylostella midgut aminopeptidase N     enzymes related to Bacillus thuringiensis toxin-binding proteins,”     Eur. J. Biochem. 248:748–761. -   Ferre, J., Real, M. D., Van Rie, J., Jansens, S. and     Peferoen, M. (1991) “Resistance to the Bacillus thuringiensis     bioinsecticide in a field population of Plutella xylostella is due     to a change in a midgut membrane receptor,” Proc. Natl. Acad. Sci.     USA 88:5119–5123. -   Fujii, H., Nakajima, M., Saiki, I., Yoneda, J., Azuma, I., and     Tsuruo, T. (1995) “Human-melanoma invasion and metastasis     enhancement by high expression of aminopeptidase N/CD 13,” Clin.     Exp. Metast. 13:337–344. -   Garczynski, S. F. and Adang, M. J. (1995) “Bacillus thuringiensis     CryIA(c) d-endotoxin binding aminopeptidase in the Manduca sexta     midgut has a glycosyl-phosphatidylinositol anchor,” Insect Biochem.     Mol. Biol. 25:409–415. -   Garczynski, S. F., Crim, J. W. and Adang, M. J. (1991)     “Identification of putative brush border membrane binding proteins     specific to Bacillus thuringiensis delta-endotoxin by protein blot     analysis,” Appl. Environ. Microbiol. 57:2816–2820. -   Gill, S., Cowles, E. A. and Francis, V. (1995) “Identification,     isolation, and cloning of a Bacillus thuringiensis CryIAc     toxin-binding protein from the midgut of the lepidopteran insect     Heliothis virescens,” J. Biol. Chem. 270:27277–27282. -   Hua, G., Tsukamoto, K., Rasilo, M. and Ikezawa, H. (1998) “Molecular     cloning of a GPI-anchored aminopeptidase N from Bombyx mori midgut:     a putative receptor for Bacillus thuringiensis CryIA toxin,” Gene     214:177–185. -   Kennard, M. L., Shimizu, K. Y., Gabathuler, R., Rothenberger, S.,     Therlmann, D. and Jefferies, W. A. (1997) “Expression of cell     surface GPI-anchored human p97 in baculovirus-infected insect     cells,” Biotech. Bioeng. 55:41–53. -   Kenny, A. J., Stephenson, S. L. and Turner, A. J. (1987) “Cell     surface peptidases,” In Kenny, A. J. and Turner, A. J. (eds.)     Mammalian ectoenzymes, Elsevier, Amsterdam, pp. 169–210. -   Knight, P. J. K., Crickmore, N. and Ellar, D. J. (1994) “The     receptor for Bacillus thuringiensis CryIA(c) delta-endotoxin in the     brush border membrane is aminopeptidase N,” Mol. Microbiol.     11:429–436. -   Knight, P. J. K., Knowles, B. H. and Ellar, D. J. (1995) “Molecular     cloning of an insect aminopeptidase N that serves as a receptor for     Bacillus thuringiensis CryIA(c) toxin,” J. Biol. Chem.     270:17765–17770. -   Kolb, A. F., Maileo, J., Heister, A. and Siddell, S. G (1996)     “Characterization of functional domains in the human coronavirus HCV     229E receptor,” J. Gen. Virol. 77:2515–2521. -   Kozak, M. (1987) “An analysis of 5′-noncoding sequences from 699     vertebrate messenger RNAs,” Nucl. Acids Res. 15:8125–8132. -   Laemmli, U. K. (1970) “Cleavage of structural proteins during the     assembly of the head of bacteriophage T4,” Nature 227:680–685. -   Look, A. T., Ashmun, R. A., Shapiro, L. H. and Peiper, S. C. (1989)     “Human myeloid plasma membrane glycoprotein CD13 (gp150) is     identical to aminopeptidase N,” J. Clin. Invest. 83:1299–1307. -   Lu, Y. and Adang, M. J. (1996) “Conversion of Bacillus thuringiensis     CryIAc-binding aminopeptidase to a soluble form: by endogenous     phosphatidylinositol phospholipase C,” Insect Biochem. Molec. Biol.     226:33–40. -   Luo, K., Lu, Y. J. and Adang, M. J. (1996) “A 106 kDa form of     aminopeptidase is a receptor for Bacillus thuringiensis CryIC     delta-endotoxin in the brush border membrane of Manduca sexta,”     Insect Biochem. Mol. Biol. 26:783–791. -   Luo, K., Sangadala, S., Masson, L., Mazza, A., Brousseau, R. and     Adang, M. J. (1997a) “The Heliothis virescens 170-kDa aminopeptidase     functions as ‘Receptor A’ by mediating specific Bacillus     thuringiensis CryIA d-endotoxin binding and pore formation,” Insect     Biochem. Mol. Biol. -   Luo, K., Tabashnik, B. E. and Adang, M. J. (1997b) “Binding of     Bacillus thuringiensis CryIAc toxin to aminopeptidase in susceptible     and resistant diamondback moths (Plutella xylostella),” Appl.     Environ. Microbiol. 63:1024–1027. -   Luo, K., McLachlin, J. R., Brown, M. R., Adang, M. J. (1999)     “Expression of a Glycosylphosphatidylinositol-Linked Manduca sexta     Aminopeptiase N in Insect Cells,” Protein Expression and     Purification 17:113–122. -   Masson, L., Lu, Y., Mazza, A., Brosseau, R. and Adang, M. J. (1995)     “The CryIA(c) receptor purified from Manduca sexta displays multiple     specificities,” J. Biol. Chem. 270:20309–20315. -   McConville, M. J. and Ferguson, M. A. J. (1993) “The structure,     biosynthesis and function of glycosylated phosphatidylinositols in     the parasitic protozoa and higher eukaryotes,” Biochemical Journal     294:305–324. -   McLaughlin, S. and Aderem, A. (1995) “The myristoyl-electrostatic     switch: a modulator of reversible protein-membrane interactions,”     TIBS 20:272–276. -   Morris, T. D. and Miller, L. K. (1992) “Promoter influence on     baculovirus-mediated gene expression in permissive and     non-permissive insect cell lines,” J. Virol. 66:7397–7405. -   Plakidou-dymock, S., Tanner, M. J. and McGivan, J. D. (1993) “A role     for aminopeptidase N in Na⁺-dependent amino acid transport in bovine     renal brush-border membranes,” Biochem. J. 290:59–65. -   Richardson, M. A., Smith, D. R. J., Kemp, D. H. and     Tellam, R. L. (1993) “Native and baculovirus-expressed forms of the     immunoprotective protein BM86 from boophilus microplus are anchored     to the cell membrane by a glycosylphosphatidyl inositol linkage,”     Insect Mol. Biol. 1:139–147. -   Sambrook et al. (1989) Molecular Cloning: A Laboratory Manual Cold     Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. -   Sangadala, S., Walters, F., English, L. H. and Adang, M. J. (1994)     “A mixture of Manduca sexta aminopeptidase and alkaline phosphatase     enhances Bacillus thuringiensis insecticidal CryIA(c) toxin binding     and ⁸⁶Rb⁺—K⁺ leakage in Vitro,” J. Biol. Chem. 269:10088–10092. -   Schwartz, J.-L., Lu, Y. J., Soehnlein, P., Brousseau, R., Masson,     L., Laprade, R. and Adang, M. J. (1997) “Ion channels formed in     planar lipid bilayers by Bacillus thuringiensis toxins in the     presence of Manduca sexta midgut receptors,” FEBS Lett. 412:270–276. -   Takasaki, S., Erickson, R. H., Kim, Y. S., Kochibe, N. and     Kobata, A. (1991) “N-linked neutral sugar chains of aminopeptidase N     purified from rat small intestinal brush-border membrane,” Biochem.     30:9102–9110. -   Tomita, M., Obara, H., Takesue, Y., Tamura, H.-O., Miyajima, s.,     Taguchi, R. and Ikezawa, H. (1994) “Purification of     glycosylphosphatidylinositol-anchoring aminopeptidase N from the     plasma membrane of larval midgut epithelial cells of the silkworm,     Bombyx mori,” Int. J. Biochem. 26:977–986. -   Towbin, H., Staehelin, T. and Gordon, J. (1979) “Electrophoretic     transfer of proteins from polyacrylamide gels to nitrocellulose     sheets: procedure and some applications,” Proc. Natl. Acad. Sci. USA     76:4350–4354. -   Tresnan, N. B., Levis, R. and Holmes, K. V. (1996) “Feline     aminopeptidase N serve as a receptor for feline, canine, porcine,     and human coronaviruses in serogroup I,” J. Virol. 70:8669–8674. -   Valaitis, A., Lee, M. K., Rajamohan, F. and Dean, D. H. (1995)     “Brush border membrane aminopeptidase-N in the midgut of the gypsy     moth serves as the receptor for the CryIA(c) d-endotoxin of Bacillus     thuringiensis,” Insect Biochem. Molec. Biol. 25:1143–1151. -   Vaughn, J. L., Goodwin, R. H., Tompkins, G. L. and     McCawley, P. (1977) “The establishment of two insect cell lines from     the insect Spodoptera frugiperda (Lepidoptera:Noctuidae),” In Vitro     Cell. Dev. Biol. 13:213–217. -   Wolfersberger, M. G, Luthy, P., Maurer, A., Parenti, P., Sacchi, V.     F., Giordana, B. and Hanozet, G. M. (1987) “Preparation and partial     characterization of amino acid transporting brush border membrane     vesicles from the larval midgut of the cabbage butterfly (Pieris     brassicae),” Comp. Biochem. Physiol. 86A: 301–308. -   Yaoi, K., Kadotani, T., Kuwana, H., Shinkawa, A., Takahashi, T.,     Iwahana, H. and Sato, M. (1997) “Aminopeptidase N from Bombyx mori     as a candidate for the receptor of Bacillus thuringiensis Cry1Aa     toxin,” Eur. J. Biochem. 246:652–657. -   Yeager, C. L., Ashmun, R. A., Williams, R. K., Cardellichio, C. B.,     Shapiro, L. H., Look, A. T. and Holmes, K. V. (1992) “Human     aminopeptidase N is a receptor for human coronavirus 229E,” Nature     357:420–422. 

1. A transgenic cell comprising an isolated polynucleotide encoding an aminopeptidase N (APN), wherein a functional APN is produced by said cell and localized on the extracellular membrane of said cell, and said APN is able to bind a molecule selected from the group consisting of a Bacillus insect toxin protein and an APN inhibitor, wherein said polynucleotide is capable of hybridizing with a probe corresponding to the complement of the nucleic acid sequence of SEQ ID NO:2, wherein hybridization is maintained under conditions of 0.1× standard saline sodium phosphate EDTA buffer (SSPE) and 0.1% sodium dodecyl sulfate (SDS) at 65° C. for 15 minutes.
 2. The transgenic cell of claim 1 wherein said APN is able to bind a Bacillus insect toxin protein.
 3. The transgenic cell of claim 1 wherein said APN is able to bind an APN inhibitor.
 4. The transgenic cell of claim 1 wherein said cell is an insect cell.
 5. A transgenic cell comprising an isolated polynucleotide encoding an aminopeptidase N (APN), wherein a functional APN is produced by said cell and localized on the extracellular membrane of said cell, and said APN is able to bind a molecule selected from the group consisting of a Bacillus insect toxin protein and an APN inhibitor, wherein said APN has at least 95% identity with the amino acid sequence of SEQ ID NO:1.
 6. A method of screening for a Bacillus insect toxin protein wherein said method comprises the steps of: a) obtaining a transgenic cell comprising a heterologous polynucleotide encoding an aminopeptidase N (APN) wherein a functional APN is produced by said cell and localized on the extracellular membrane of said cell, and said APN is able to bind a Bacillus insect toxin protein; b) exposing said cell to a composition suspected of comprising a Bacillus insect toxin; and c) determining if a Bacillus insect toxin is bound to said APN, wherein said polynucleotide is capable of hybridizing with a probe corresponding to the complement of the nucleic acid sequence of SEQ ID NO:2, wherein hybridization is maintained under conditions of 0.1× standard saline sodium phosphate EDTA buffer (SSPE) and 0.1% sodium dodecyl sulfate (SDS) at 65° C. for 15 minutes.
 7. The method of claim 6 wherein said APN has at least 95% identity with the amino acid sequence of SEQ ID NO:1.
 8. A method of screening for an aminopeptidase inhibitor wherein said method comprises the steps of: a) obtaining a plurality of transgenic cells wherein each said cell comprises an isolated polynucleotide encoding an aminopeptidase N (APN), wherein a functional APN is produced by said cell and localized on the extracellular membrane of said cell, and said APN is able to bind an aminopeptidase inhibitor; b) exposing said cells to at least one potentially APN-inhibiting compound; c) identifying a compound that binds to said APN; and d) assaying said compound for its ability to inhibit an aminopeptidase, wherein said polynucleotide is capable of hybridizing with a probe corresponding to the complement of the nucleic acid sequence of SEQ ID NO:2, wherein hybridization is maintained under conditions of 0.1× standard saline sodium phosphate EDTA buffer (SSPE) and 0.1% sodium dodecyl sulfate (SDS) at 65° C. for 15 minutes.
 9. The method of claim 8 wherein said compound binds to an aminopeptidase of an insect.
 10. The method of claim 8 wherein said compound inhibits an aminopeptidase of an insect.
 11. The method of claim 8 wherein said APN has at least 95% identity with the amino acid sequence of SEQ ID NO:1. 